The desired alignment could be achieved by setting the TextAlignment property of the paragraph to RadTextAlignment.Justify, just as you have done in the commented part in your example code.
paragraph.TextAlignment = RadTextAlignment.Justify;
That alignment will be applied for each newly added paragraph after the one you've set the property to.
In case you need to align the text in an existing document, you need to select the whole document, apply the alignment and then eventually clear the selection. Here is the code snippet for this:
Setting RichTextBox.HorizontalAlignment property to HorizontalAlignment.Stretch is not recommended as it could lead to issues with the formatting.
I hope this helps.
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